For LCD devices, a classification based on the operating mode for liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode, and a field-induced photo-reactive alignment (FPA) mode. A classification based on the driving mode of the device includes passive matrix (PM) and active matrix (AM) types. The PM type is classified into static type, multiplex type and so forth, and the AM type is classified into thin film transistor (TFT) type, metal insulator metal (MIM) type and so forth. The TFT type is further classified into amorphous silicon type and polysilicon type. The latter is classified into a high temperature type and a low temperature type according to the production process. A classification based on the light source includes a reflective type utilizing natural light, a transmissive type utilizing a backlight and a transflective type utilizing both natural light and a backlight.
An LCD device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving the characteristics of the composition. Table 1 below summarizes the relationship between the characteristics of two aspects. The characteristics of the composition will be further described based on a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher and a preferred minimum temperature of the nematic phase is about −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred.
TABLE 1Characteristics of Composition and AM DeviceNo.Characteristics of CompositionCharacteristics of AM Device1Wide temperature range ofWide usable temperature rangea nematic phase2Small viscosityShort response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage anddielectric anisotropysmall electric powerconsumptionLarge contrast ratio5Large specific resistanceLarge voltage holding ratio andlarge contrast ratio6High stability to UVLong service lifelight and heat
The optical anisotropy of the composition relates to the contrast ratio of the device. According to the mode of the device, a suitable optical anisotropy such as a large optical anisotropy or small optical anisotropy is required. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. In a device having the VA mode, the suitable value is in the range of about 0.30 μm to about 0.40 μm, and in a device having the IPS mode or the FFS mode, the suitable value is in the range of about 0.20 μm to about 0.30 μm. In the above cases, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large value of dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large value of dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase even after the device has been used for a long period of time is preferred. The stability of the composition to UV light and heat relates to the service life of the device. In a case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth.
A liquid crystal composition containing a polymer is used for an LCD device having a PSA mode. First, a composition to which a small amount of polymerizable compound is added is injected into the device. Next, the composition is irradiated with UV light, while a voltage is applied between substrates of the device, to polymerize the polymerizable compound to forma network structure of the polymer in the composition. In the composition, alignment of liquid crystal molecules can be controlled by the polymer, and therefore the response time of the device is shortened and image persistence is reduced. Such an effect of the polymer can be expected for a device having a mode such as the TN, ECB, OCB, IPS, VA, FFS or FPA mode.
A composition having a positive dielectric anisotropy (Δ∈) is used for an AM device having the TN mode. A composition having a negative Δ∈ is used for an AM device having the VA mode. A composition having a positive or negative Δ∈ is used for an AM device having the IPS or FFS mode. A composition having a positive or negative Δ∈ is used for an AM device having the PSA mode. Examples of the liquid crystal composition having the negative Δ∈ are disclosed in Patent literature Nos. 1 to 3 described below.